Now, Horford still is trying to wrap his head around the fact that he will be lining up against Boston on Wednesday night as a member of the Sixers in the teams' season opener.
"It is very surreal," Horford said after practice Tuesday. "I never would have imagined being in this position. For me, it's a great challenge ahead, being here and trying to accomplish something special here."
Horford entered last season expecting to be part of something special in Boston. The Celtics had sky-high expectations after making it to the Eastern Conference finals the year before with both Kyrie Irving and Gordon Hayward sidelined due to injury.
But rather than taking the East by storm last season, the Celtics instead fell flat, finishing with the fourth seed before being steamrolled by the top-seeded Milwaukee Bucks in five games in the second round.
Philadelphia enters this season with similar expectations after a busy summer that saw Horford and Josh Richardson arrive, and both Ben Simmons and Tobias Harris get massive long-term deals. Horford says he learned a few things from the way last season played out in Boston that he hopes to translate to this season with the Sixers.
"It's good to have expectations, I believe," he said. "You want to come out, you want to compete, you want to do special things. People see we are capable of them, but we have to go out and do them.
"One thing I've learned is that just because you're expected [to win] doesn't mean you're going to get it done. I do believe we have a lot of work ahead as a group, we just do, and we'll take it game-by-game this season."
Horford surprised many by leaving Boston as a free agent this summer after opting out of the final year of his contract in June. For all of the attention that Irving's departure to Brooklyn has received, the Celtics were able to replace him with another All-Star point guard, Kemba Walker. Boston didn't have the same ability to replace Horford with a like replacement, and instead enters Wednesday's showdown with a decidedly unsettled center rotation in his absence.
That, however, is not Horford's concern. And while he won't play the Celtics in Boston until Dec. 12, he said he is happy to get the first meeting out of the way as fast as he could.
"It's a great opportunity for me," Horford said. "I'm definitely put in a weird position here, but playing it now, to your question, it's the best thing for me to go out there and do it and really cement myself here in Philadelphia."
It also will be Horford's first opportunity to play alongside Joel Embiid in a regular-season game. Horford earned a deserved reputation as one of few defenders capable of slowing down the Sixers' superstar center while playing for the Celtics, but now will be playing much more regularly as a power forward alongside Embiid.
"It's a different challenge," Horford said. "For me, I'll be guarding more on the perimeter, doing different things. It's a different challenge, and I welcome and accept it, and I am glad I'm not guarding him anymore."